Integrated circuit fabrication is typically accomplished by forming many different layers on a substrate. As used herein, the phrase integrated circuit refers to circuits such as those formed on monolithic substrates of a semiconducting material, such as group IV materials like silicon and germanium, and group III-V compounds such as gallium arsenide. Because the design tolerances of an integrated circuit are so strict, it is desirable to monitor the properties, such as thickness and elemental composition, of the various layers as they are formed. One way to measure the properties of film layers is to use electron microprobe x-ray spectrometry.
Electron microprobe x-ray spectrometry uses an electron beam source to excite a sample. X-rays having wavelengths that are characteristic of the elements of the sample are emitted from the sample over a continuous range of takeoff angles, defined as the angle between the x-ray and the sample surface. An x-ray detector assembly is positioned to detect a fraction of the x-rays that are emitted from the sample. The detector assembly can capture x-rays emitted over a finite range of takeoff angles. The detector assembly includes both a spectrometer and an x-ray detector. The spectrometer selects x-rays within a narrow range of wavelengths and directs only those x-rays to the x-ray detector. This is typically accomplished by rotating a diffractor through a range of angles, where at each angular position of the diffractor, the diffractor deflects x-rays with a given wavelength range towards the detector. The rate of impingement of the x-rays within subsets of the desired range of wavelengths is sequentially detected and measured. From this information, properties such as the elemental composition and thickness of the sample can eventually be determined.
There are several methods of detecting the x-rays with different energies. A first method uses a set of curved crystal or multilayer reflectors attached to a rotatable turret. As used herein, a “turret” refers to a carousel-like holder of one or more objects, such as reflectors, disposed in a circumferential relationship to each other with respect to some axis of rotation and positions the objects at a particular location by rotation of the holder about the axis. One reflector is positioned so that it reflects and focuses x-rays through an aperture into a gas proportional counter. The gas proportional counter converts the x-ray into an electrical pulse that is detected by an electronic detection system. Each of the reflectors on the turret can reflect and focus x-rays over a fixed Bragg energy range. At a particular position and orientation, a first reflector can reflect and focus x-rays over a first narrow energy range. This first narrow energy range is of the right size to capture the characteristic x-rays emitted by a first element. The first reflector can be rotated and repositioned to reflect x-rays emitted from a second element contained within a second narrow range within the first reflector's Bragg range. In order to capture x-rays from a third element not contained with the first reflector's Bragg range, it is necessary to rotate the turret to bring a second reflector into position. This reflector can reflect and focus the x-rays from a third element contained within a third narrow energy range and within the second reflector's Bragg range. The union of all the reflectors' Bragg energy ranges determines how many elements can be detected.
If enough detectors are included, and if they have overlapping Bragg energy ranges, it is possible to detect almost all elements in the periodic table. In addition, it is often desirable to measure the x-rays with energies on either side of the narrow range emitted from the element being measured. This can be accomplished by periodically rotating and repositioning the reflector by small amounts, and recording the number of x-rays at each position. In other words, the detector is scanned in energy. In this way the x-ray spectrum can be measured in the energy region around and including the element's narrow range. This is useful for determining the background x-ray intensity that is later subtracted from the elemental narrow range measurement to get the true x-ray intensity emitted from the element.
There are several drawbacks to the above method. For example, in order to capture the x-rays with high efficiency it is desirable for the reflector to intercept the x-rays with as large a solid angle as possible. A large solid angle for collecting x-rays allows measurements to be made quickly. This requires either positioning the reflector close to the x-ray source, or using a very large area reflector. Unfortunately, the turret design places geometrical restrictions on the number of large solid angle reflectors that can be accommodated by the turret. Commercially available systems typically accommodate from two to a maximum of six reflectors. It is desirable to have a larger number of reflectors, both to be able to cover a large number of elements and to optimize the efficiency for each element.
Prior art x-ray detection systems suffer from other drawbacks. For example, in order to have a reflector with a moderately large Bragg energy range, or to be able to scan it to measure a spectrum, it is necessary for it to be curved in a circular cylindrical shape. An inside surface of the cylindrical reflector has multilayer dielectric coating having a d-spacing that is constant over a surface of the dielectric. The reflector, source (e.g., sample surface) and detector are located at points on a circle known as the Rowland circle such that the detector stays at a linear focus of the reflector. The Rowland circle has a diameter that is half the diameter of the cylindrical reflector. Unfortunately, a circular cylindrical reflector is not an optimum shape to achieve the best efficiency for a particular element. In addition, the solid angle for collecting x-rays is limited to about 0.03 steradians.
Attempts have been made to improve the efficiency of cylindrical reflectors. One technique uses a multilayer d-spacing having a gradient across the reflector to keep the efficiency high. Unfortunately, the optic cannot be scanned with much efficiency and the solid angle for collection of x-rays is limited to about 0.10 steradians. Another technique uses a three-dimensional ellipsoidal multilayer optic that can focus x-rays from a point source to a small spot image. Unfortunately, the ellipsoidal shape is difficult and expensive to manufacture. In addition. the ellipsoidal multilayer optic also uses a multilayer dielectric with a graded d-spacing. Consequently, the optic cannot be scanned.
What is needed, therefore, are large collection angle x-ray monochromators that can overcome some of the problems described above.